ClosestByNetworkDistanceRule

Sends whichever idle transporter has the shortest journey to the pickup.

The default, and the obvious policy: it minimises the empty running that a fleet does. Note that distance here is distance along the guide path, not separation in space. On a one-way loop a transporter standing a few feet past the pickup point may have to go all the way round, so choosing by straight-line distance would routinely send the wrong one -- quietly, and with no symptom other than a fleet that performs worse than it should.

Ties go to the earlier transporter in the candidate list, which is declaration order, so the choice is the same on every run.